Lithium secondary battery and preparation thereof

ABSTRACT

The present invention provides a lithium secondary battery and the preparation thereof, more specifically a lithium secondary battery comprising an electrode assembly having a cathode, an anode, and a separator interposed between the cathode and the anode; and a non-aqueous electrolyte solution impregnated in the electrode assembly, wherein the separator further comprises a layer having a plurality of destroyed capsules dispersed therein, the layer being formed on at least one surface of the separator coming into contact with the cathode and the anode, and the destroyed capsules has a film formed from a binder polymer and inorganic particles dispersed therebetween. The lithium secondary battery of the present invention can be prepared without the separate introducing process of a non-aqueous electrolyte solution, and has a separator exhibiting improved mechanical property and safety.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of International ApplicationNo. PCT/KR2012/011622 filed on Dec. 27, 2012, which claims priority toKorean Patent Application No. 10-2011-0143839 filed in the Republic ofKorea on Dec. 27, 2011, and Korean Patent Application No.10-2012-0154703 filed on Dec. 27, 2012, the disclosures thereof areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a lithium secondary battery and amethod for the preparation thereof, more specifically to a lithiumsecondary battery which comprises a separator having enhanced mechanicalproperty and can be simply prepared without the introducing process of anon-aqueous electrolyte solution, and a method for the preparationthereof.

BACKGROUND ART

Recently, there has been growing interest in energy storagetechnologies. As energy storage technologies are extended to devicessuch as cellular phones, camcorders and notebook PC, and further toelectric vehicles, demand for high energy density of batteries used as asource of power supply of such devices is increasing. Therefore,research and development of lithium secondary batteries, which most meetthe demand, are actively being conducted.

Among secondary batteries currently used, a lithium secondary batterydeveloped in the early 1990's comprises an anode made of carbon materialcapable of intercalating or disintercalating lithium ions, a cathodemade of lithium-containing oxide, and a non-aqueous electrolyte solutionobtained by dissolving a suitable amount of lithium salt in a mixedorganic solvent, and is prepared by storing such components in a case.

Such a lithium secondary battery is conventionally prepared by insertingan electrode assembly having electrodes on both sides of a separator ina battery case, and introducing a non-aqueous electrolyte solution intothe battery case.

However, in order to introduce the non-aqueous electrolyte solution inthe battery case, the battery case needs to be cut before introductionand then be closed after introduction, which brings about a great amountof inconvenience and may also damage the materials of the battery case.Until now, there has been no efficient solution for such a problem.

Meanwhile, overheating of lithium secondary batteries may cause thermalrunaway or a puncture of a separator may pose an increased risk ofexplosion. In particular, porous polyolefin substrates commonly used asseparators of lithium secondary batteries undergo severe thermalshrinkage at a temperature of 100° C. or higher in view of theirmaterial characteristics and production processes including elongation.This thermal shrinkage behavior may cause a short circuit between acathode and an anode.

Accordingly, various researches have been made in order to prevent sucha short circuit between a cathode and an anode even though batteriesmalfunction.

DISCLOSURE Technical Problem

The present invention is designed to solve the above-mentioned problems,and therefore it is an object of the present invention to provide alithium secondary battery whose non aqueous electrolyte can beintroduced in a simple way, which can prevent a short circuit between acathode and an anode even though batteries malfunction, and a method forthe preparation thereof.

Technical Solution

In order to accomplish the above object, in accordance with one aspectof the present invention, there is provided a lithium secondary battery,comprising an electrode assembly having a cathode, an anode, and aseparator interposed between the cathode and the anode; and anon-aqueous electrolyte solution impregnated in the electrode assembly,wherein the separator further comprises a layer having a plurality ofdestroyed capsules dispersed therein, the layer being formed on at leastone surface of the separator coming into contact with the cathode andthe anode, and the destroyed capsules has a film formed from a binderpolymer and inorganic particles dispersed therebetween.

In the lithium secondary battery of the present invention, the inorganicparticles are preferably selected from the group consisting of inorganicparticles having a dielectric constant of 5 or higher, inorganicparticles having the ability to transport lithium ions, and a mixturethereof.

In the lithium secondary battery of the present invention, the binderpolymer is preferably selected from the group consisting ofpolyethylene, polystyrene, polyvinylidenefluoride-co-hexafluoropropylene, polyvinylidenefluoride-co-trichloroethylene, polyethylene glycol diacrylate,polyethylene glycol phosphate diacrylate, polyacrylate, polymethylmethacrylate, polyisobutylmethyl methacrylate, polybutyl acrylate,polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, polyvinylalcohol, polyethylene-co-vinyl acetate, polyethylene oxide, polyarylate,polyteterahydrofuran, polymethacrylic acid lithium, polyacrylic acidlithium, polymaleic acid lithium, polyvinyl sulfonic acid lithium,polyvinyl phosphonic acid lithium, cellulose acetate, cellulose acetatebutyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan,carboxymethyl cellulose, and a mixture thereof, but the presentinvention is not limited thereto.

In the lithium secondary battery of the present invention, the weightratio of the inorganic particles and the binder polymer may be in therange of 1:1 to 10:1.

In the lithium secondary battery of the present invention, the capsulestores a non-aqueous electrolyte solution before being destroyed.

In accordance with another aspect of the present invention, there isprovided a method for preparing a lithium secondary battery, comprisingapplying a plurality of capsules and a dispersion medium on at least onesurface of a separator or on a surface of at least one of a cathode andan anode to come into contact with the separator, the capsules having afilm formed from a binder polymer and inorganic particles dispersedtherebetween and storing a non-aqueous electrolyte solution; forming anelectrode assembly by interposing the separator between the cathode andthe anode; inserting the electrode assembly in a battery case; andbringing the electrode assembly into thermocompression before, after orboth the electrode assembly is inserted in the battery case, to destroythe capsules and impregnate the non-aqueous electrolyte solution intothe electrode assembly.

In the method of the present invention, the capsules may be applied onall of the separator, the cathode and the anode.

Advantageous Effects

The lithium secondary battery of the present invention, which isprepared by using capsules storing a non-aqueous electrolyte solution sothat during thermocompression of an electrode assembly, the non-aqueouselectrolyte solution may be uniformly provided to the entire area of theelectrode assembly without a separate process, which improves wettingproperty of the electrolyte solution to electrodes. In accordance withthe present invention, a conventional process of introducing anon-aqueous electrolyte solution is not necessary, and thus, apreparation of batteries can be simplified.

Also, according to the present invention, the cathode of the battery maybe applied with capsules storing an anti-oxidizing agent on the surfacethereof, and the anode of the battery may be applied with capsulesstoring an anti-reductive agent on the surface thereof, therebyproviding an electrochemically more stable electrolyte. In addition,when the capsules are applied on a separator, they are selectivelyapplied on the interface of the separator and the cathode or on theinterface of the separator and the anode, thereby minimizing thedecomposition of the electrolyte during the operation of the battery.

Further, since the lithium secondary battery of the present invention isprepared by using capsules whose film is made of inorganic particles anda binder polymer, destroyed capsules can strengthen the mechanicalproperty of the separator and can prevent a short circuit between acathode and an anode even though batteries malfunction.

DESCRIPTION OF DRAWINGS

Other objects and aspects of the present invention will become apparentfrom the following descriptions of the embodiments with reference to theaccompanying drawings in which:

FIG. 1 is a schematic cross-sectional view of a unit electrode assemblycomprised in a lithium secondary battery according to one embodiment ofthe present invention.

FIG. 2 is a SEM photograph of capsules obtained in one embodiment of thepresent invention.

FIG. 3 is a SEM photograph showing the surface of an anode before athermocompression process according to one embodiment of the presentinvention.

FIG. 4 is a SEM photograph showing the surface of an anode after athermocompression process according to one embodiment of the presentinvention.

FIG. 5 is a graph showing a thickness change after fully chargingbatteries obtained in Example 1 and Comparative Example 1, followed bystoring at 85° C. for 4 hours.

FIG. 6 is a graph showing discharging voltage profiles at 1 C forbatteries obtained in Example 1 and Comparative Example 1.

FIG. 7 is a graph showing the results of a high rate discharge test at10 C for batteries obtained in Example 1 and Example 2.

BEST MODE

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings. Priorto the description, it should be understood that the terms used in thespecification and the appended claims should not be construed as limitedto general and dictionary meanings, but interpreted based on themeanings and concepts corresponding to technical aspects of the presentinvention on the basis of the principle that the inventor is allowed todefine terms appropriately for the best explanation.

FIG. 1 schematically shows a unit electrode assembly according to oneembodiment of the present invention. However, the configurationsillustrated in the drawings and the embodiments are just preferableexamples for the purpose of illustrations only, not intended to limitthe scope of the disclosure, so it should be understood that otherequivalents and modifications could be made thereto without departingfrom the spirit and scope of the disclosure.

Referring to FIG. 1, the present invention provides an electrodeassembly having a cathode 20, an anode 30, and a separator 10. Theseparator 10 may comprise a layer 50 having a plurality of destroyedcapsules 51 dispersed therein, the layer being formed on a part of theseparator 10 coming into contact with the cathode 20, a part of theseparator 10 coming into contact with the anode 30, or both. FIG. 1illustrates a case in which the separator 10 comprises the layer 50having a plurality of destroyed capsules 51 dispersed therein on bothparts of the separator 10 coming into contact with the cathode 20 andthe anode 30.

In the lithium secondary battery of the present invention, the capsules51 store a non-aqueous electrolyte therein before their destruction andform a layer on at least one surface of the separator coming intocontact with electrodes. The capsules 51 are destroyed when heat orpressure is applied to the electrode assembly during the preparation ofthe battery, while the stored non-aqueous electrolyte (not shown) isimpregnated in the separator and at least one electrode of the cathodeand the anode, by which the only film of the capsules remains on atleast one surface of the separator coming into contact with at least oneelectrode of the cathode and the anode to form the layer 50.

In the present invention, the film of the capsules is formed from abinder polymer and inorganic particles. The binder polymer of thecapsule film forms a bond with the separator after the capsules aredestroyed by thermocompression, thereby enhancing the mechanicalstrength of the separator. The inorganic particles act as a spacercapable of maintaining the physical form of the destroyed capsule layer50 to hinder the heat shrinkage of a porous substrate when anelectrochemical device overheats or to prevent a short circuit betweenelectrodes when thermal runaway occurs.

Also, interstitial volumes are present between the inorganic particlesto form micropores. That is, in the destroyed capsule layer 50, thebinder polymer allows the adhesion of the inorganic particles so thatthe inorganic particles can be bound with each other (i.e., the binderpolymer connects and immobilizes the inorganic particles therebetween),and also, the destroyed capsule layer 50 maintains the bonding statewith a porous substrate by the binder polymer. In the destroyed capsulelayer 50, the inorganic particles are substantially present in contactwith each other to form a closest packed structure, and an interstitialvolume generated from the contact of the inorganic particles with eachother becomes a pore of a porous coating layer.

In addition, since the destroyed capsules do not form one layersuccessively but a plurality of capsules are dispersed to form onelayer, an interstitial volume present between the destroyed capsules 51may become a pore of the destroyed capsule layer 50.

In the lithium secondary battery of the present invention, the inorganicparticles which form the film of the capsules are not particularlylimited if they are electrochemically stable. That is, the inorganicparticles which may be used in the present invention are notparticularly limited unless an oxidation-reduction reaction occurs in anoperating voltage range (for example, 0 to 5 V based on Li/Li⁺) of anapplied electrochemical device. Particularly, inorganic particles havinga high dielectric constant may be used to increase a dissociation rateof an electrolyte salt, e.g., a lithium salt, in a liquid electrolyte,thereby improving an ionic conductivity of the electrolyte.

For the foregoing reasons, the inorganic particles used in the presentinvention preferably include inorganic particles having a dielectricconstant of 5 or higher, preferably 10 or higher.

Non-limiting examples of the inorganic particles having a dielectricconstant of 5 or higher include BaTiO₃, Pb(Zr,Ti)O₃ (PZT),Pb_(1-x)La_(x)Zr_(1-y)Ti_(y)O₃ (PLZT, 0<x<1, 0<y<1),Pb(Mg_(1/3)Nb_(2/3))O₃PbTiO₃ (PMN-PT), hafnia (HfO₂), SrTiO₃, SnO₂,CeO₂, MgO, NiO, CaO, ZnO, ZrO₂, SiO₂, Y₂O₃, Al₂O₃, SiC, TiO₂ inorganicparticles and a mixture thereof.

Also, as the inorganic particles, inorganic particles having the abilityto transport lithium ions, i.e., lithium-containing inorganic particleswhich can transfer lithium ions without holding them, may be used.Non-limiting examples of the inorganic particles having the ability totransport lithium ions include lithium phosphate (Li₃PO₄), lithiumtitanium phosphate (Li_(x)Ti_(y)(PO₄)₃, 0<x<2, 0<y<3), lithium aluminumtitanium phosphate (Li_(x)Al_(y)Ti_(z)(PO₄)₃, O<x<2, 0<y<1, 0<z<3),(LiAlTiP)_(x)O_(y) type glass (0<x<4, 0<y<13) such as14Li₂O-9Al₂O₃-38TiO₂-39P₂O₅, lithium lanthanum titanate(Li_(x)La_(y)TiO₃, 0<x<2, 0<y<3), lithium germanium thiophosphate(Li_(x)Ge_(y)P_(z)S_(w), 0<x<4, 0<y<1, 0<z<1, 0<w<5) such asLi_(3.25)Ge_(0.25)P_(0.75)S₄, lithium nitride (Li_(x)N_(y), 0<x<4,0<y<2) such as Li₃N, SiS₂ type glass (Li_(x)Si_(y)S_(z), 0<x<3, 0<y<2,0<z<4) such as Li₃PO₄—Li₂S—SiS₂, P₂S₅ type glass (Li_(x)P_(y)S_(z),0<x<3, 0<y<3, 0<z<7) such as LiI—Li₂S—P₂S₅, and a mixture thereof.

Further, in the present invention, the binder polymer used in thecapsules has preferably a glass transition temperature (T_(g)) of −200to 200° C. so as to improve the mechanical properties such asflexibility and elasticity of the coating layer finally formed.

Also, the binder polymer is not necessarily required to have ionicconductivity, however, a polymer having ionic conductivity may be usedto improve the performances of electrochemical devices. Accordingly, thebinder polymer used in the present invention preferably includes onehaving a high dielectric constant. Actually, the dissociation rate of asalt in an electrolyte solution depends on a dielectric constant of theelectrolyte solution. Therefore, the higher the dielectric constant ofthe binder polymer, the more the dissociation rate of a salt in anelectrolyte solution increases. In this regard, in the presentinvention, the binder polymer may have a dielectric constant of 1.0 to100 (measuring frequency=1 kHz), preferably 10 or higher.

Non-limiting examples of the binder polymer include polyethylene,polystyrene, polyvinylidene fluoride-co-hexafluoropropylene,polyvinylidene fluoride-co-trichloroethylene, polyethylene glycoldiacrylate, polyethylene glycol phosphate diacrylate(copolymer ofpolyethyleneglycol diacrylate and bis[2-acryloyloxyethyl]phosphate),polyacrylate, polymethyl methacrylate, polyisobutylmethyl methacrylate,polybutyl acrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyvinyl alcohol, polyethylene-co-vinyl acetate, polyethyleneoxide, polyarylate, polyteterahydrofuran, polymethacrylic acid lithium,polyacrylic acid lithium, polymaleic acid lithium, polyvinyl sulfonicacid lithium, polyvinyl phosphonic acid lithium, cellulose acetate,cellulose acetate butyrate, cellulose acetate propionate,cyanoethylpullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose,cyanoethyl sucrose, pullulan, carboxymethyl cellulose, and a mixturethereof.

The inorganic particles and the binder polymer are preferably used in aweight ratio of 1:1 to 10:1, more preferably 1.2:1 to 9:1. When theweight ratio of the inorganic particles and the binder polymer satisfiessuch a range, the dispersibility of the inorganic particles in thecapsules storing the non-aqueous electrolyte solution can be maintainedand the mechanical strength of the film of the capsules can be improved.

The non-aqueous electrolyte solution stored in the capsules may be anyone which is conventionally used in the art.

The non-aqueous electrolyte solution used in the present inventioncomprises a lithium salt as an electrolyte salt. The lithium salt may beany one which is conventionally used in an electrolyte solution for alithium secondary battery. For example, an anion of the lithium salt maybe any one selected from the group consisting of F⁻, Cl⁻, Br⁻, I⁻, NO₃⁻, N(CN)₂ ⁻, BF₄ ⁻, ClO₄ ⁻, PF₆ ⁻, (CF₃)₂PF₄ ⁻, (CF₃)₃PF₃ ⁻, (CF₃)₄PF₂⁻, (CF₃)₅PF⁻, (CF₃)₆P⁻, CF₃SO₃ ⁻, CF₃CF₂SO₃ ⁻, (CF₃SO₂)₂N⁻, (FSO₂)₂N⁻,CF₃CF₂(CF₃)₂CO⁻, (CF₃SO₂)₂CH⁻, (SF₅)₃C⁻, (CF₃SO₂)₃C⁻, CF₃(CF₂)₇SO₃ ⁻,CF₃CO₂ ⁻, CH₃CO₂ ⁻, SCN⁻ and (CF₃CF₂SO₂)₂N⁻.

The non-aqueous electrolyte solution used in the present inventioncomprises an organic solvent which is conventionally used in anelectrolyte solution for a lithium secondary battery, for example,propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate(DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methylpropyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile,dimethoxy ethane, diethoxy ethane, vinylene carbonate, sulfolane,γ-butyrolactone, propylene sulfite, teterahydrofuran, and a mixturethereof. In particular, among the above carbonate-based organicsolvents, the cyclic carbonates such as ethylene carbonate and propylenecarbonate have a high viscosity and a high dielectric constant to moreeasily dissociate a lithium salt in an electrolyte. Such a cycliccarbonate may be mixed with a linear carbonate with low viscosity andlow dielectric constant such as dimethyl carbonate and diethyl carbonatein a suitable ratio to provide an electrolyte solution with a highelectric conductivity.

The capsules storing the non-aqueous electrolyte solution may beprepared by various methods known in the art, for example, solventevaporation, coacervation, interfacial polycondensation, in-situpolymerization, micro-reaction process, piezoelectric process, spraydrying, etc.

Specifically, the capsules may be prepared by dissolving acellulose-based compound in a ketone- or ester-based solvent to obtainan organic solution; adding and dispersing an electrolyte solution inthe organic solution to obtain a mixed solution; mixing the mixedsolution with a solution of a water-soluble polymer such as polyvinylalcohols, followed by rotating at a high speed to diffuse the organicsolution and to form an emulsion; and forming microcapsules from theemulsion.

Alternatively, the capsules may be prepared by adding and uniformlydispersing an electrolyte solution to a solution of a polymer such as acellulose-based compound, polyethylene, polystyrene and polyvinylidenefluoride-co-hexafluoropropylene; spraying the polymer solutioncontaining the electrolyte solution dispersed therein by way of aconventional spraying; and maintaining the pressure and temperature ofthe reaction container properly to form capsules in the form of amicrobead.

Furthermore, the capsules may be prepared in the form of microcapsulesby using a material capable of forming bubbles by way of dispersing amicelle or colloid having hydrophilic or non-hydrophilic property. Also,the capsules may be prepared by applying an encapsulating process usedin a drug delivery system.

The above-mentioned methods for preparing capsules are presented for thepurpose of illustration, so the present invention is not limitedthereto. Further, in the present invention, the capsules containinorganic particles, to which the capsules are prepared by adding theinorganic particles to the polymer solution for the capsule film.

The capsules thus prepared may generally have a micro-size of several totens of μm, but the size of the capsules may be varied depending on theamount of the stored electrolyte solution and the kinds of a materialforming the capsule film. Also, the size and shape of the capsules maybe varied depending on the desired specific use thereof and thepreparation procedures thereof.

Hereinafter, a method for preparing a lithium secondary battery by usingthe above-mentioned capsules according to one embodiment of the presentinvention will be described. However, such a preparation method is justfor illustration, so the present invention is not limited thereto.

First, the capsules containing a non-aqueous electrolyte solutionaccording to the present invention are prepared and dispersed in aproper dispersion medium, followed by applying on at least one surfaceof a separator or on a surface of an electrode coming into contact withthe separator. The application of the resulting solution may be carriedout on both the separator and the electrode.

The separator may be obtained from a porous polymer substrate which isconventionally used as a separator in the art, for example, a porouspolymer substrate made of polyolefin-based polymers, or a poroussubstrate made of any one electrode from the group consisting ofpolyethylene terephthalate, polybutylene terephthalate, polyester,polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone,polyether sulfone, polyphenylene oxide, polyphenylene sulfite,polyethyle naphthalene and a mixture. The porous polymer substrate madeof polyolefin-based polymers may be any one which is conventionallyused. More specifically, a membrane or non-woven fabric made of apolyolefin-based polymer selected from polyethylene such as high-densitypolyethylene, linear low-density polyethylene, low-density polyethyleneand ultra-high molecular weight polyethylene, polypropylene,polybutylene, polypentene and a mixture thereof.

Also, in the present invention, the cathode and anode may be any onewhich is conventionally used in the preparation of a lithium secondarybattery.

In the lithium secondary battery of the present invention, as thecathode active material, a lithium-containing transition metal oxide maybe preferably used, for example, any one selected from the groupconsisting of Li_(x)CoO₂(0.5<x<1.3), Li_(x)NiO₂(0.5<x<1.3),Li_(x)MnO₂(0.5<x<1.3), Li_(x)Mn₂O₄(0.5<x<1.3),Li_(x)(Ni_(a)Co_(b)Mn_(c))O₂(0.5<x<1.3, 0<a<1, 0<b<1, 0<c<1, a+b+c=1),Li_(x)Ni_(1-y)Co_(y)O₂(0.5<x<1.3, 0<y<1),Li_(x)Co_(1-y)Mn_(y)O₂(0.5<x<1.3, 0≦y<1),Li_(x)Ni_(1-y)Mn_(y)O₂(0.5<x<1.3, 0≦y<1),Li_(x)(Ni_(a)Co_(b)Mn_(c))O₄(0.5<x<1.3, 0<a<2, 0<b<2, 0<c<2, a+b+c=2),Li_(x)Mn_(2-z)Ni_(z)O₄(0.5<x<1.3, 0<z<2),Li_(x)Mn_(2-z)Co_(z)O₄(0.5<x<1.3, 0<z<2), Li_(x)CoPO₄(0.5<x<1.3),LixFePO₄(0.5<x<1.3) and a mixture thereof may be used. Thelithium-containing transition metal oxide may be coated with a metalsuch as aluminum (Al) and a metal oxide. In addition, lithium-containingtransition metal sulfide, selenide, or halide may also be used.

Among these, a mixture of Li_(x)CoO₂(0.5<x<1.3) andLi_(x)(Ni_(a)Co_(b)Mn_(c))O₂ (0.5<x<1.3, 0<a<1, 0<b<1, 0<c<1, a+b+c=1),or Li_(x)CoO₂(0.5<x<1.3) coated with aluminum is preferably used as thecathode active material. Particularly, Li_(x)(Ni_(a)Co_(b)Mn_(c))O₂(0.5<x<1.3, 0<a<1, 0<b<1, 0<c<1, a+b+c=1) is preferred in terms ofproviding high output characteristics under the condition of a highvoltage.

As the anode active material, a carbon-based material, metallic lithium,silicone or tin which can conventionally intercalate and disintercalatelithium ions may be used, and also a metal oxide such as TiO₂ and SnO₂having a potential to lithium less than 2V may be used. Among these, thecarbon-based material is preferably used, and the carbon-based materialmay be low-crystalline carbon or high-crystalline carbon. Representativeexamples of the low-crystalline carbon include soft carbon and hardcarbon, and representative examples of the high-crystalline carboninclude natural graphite, Kish graphite, pyrolytic carbon, mesophasepitch based carbon fiber, meso-carbon microbeads, mesophase pitches, andhigh-temperature sintered carbon such as petroleum or coal tar pitchderived cokes.

The cathode and anode may comprise a binder, and as the binder, variouskinds of binder polymers including vinylidenefluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidenefluoride, polyacrylonitrile and polymethylmethacrylate may be used.

After the capsules are applied, the separator is interposed between thecathode and the anode to form an electrode assembly, and the electrodeassembly is inserted in a battery case.

In this case, a step for destroying the capsules to impregnate thestored non-aqueous electrolyte solution into the electrode assembly isfurther carried out, and such a step for the destruction of capsules andthe impregnation into the electrode assembly may be classified intothree ways depending on that a thermocompression procedure fordestroying the capsules is made before the insertion of the electrodeassembly in a battery case, after the insertion of the electrodeassembly, or both before and after the insertion of the electrodeassembly.

The first way is to bring the electrode assembly into thermocompressionbefore the electrode assembly is inserted in a battery case, to destroythe capsules and impregnate the stored non-aqueous electrolyte solutioninto the electrode assembly.

The destruction of the capsules containing the non-aqueous electrolytemay be carried out under the conventional thermocompression conditionsof the electrode assembly in various ranges depending on the kinds of aspecific material forming the capsule film.

After the capsules are destroyed, since the stored non-aqueouselectrolyte solution is discharged and impregnated into the separatorand the electrodes, a separate process for introducing the non-aqueouselectrolyte solution is not necessary.

Thus, after thermocompression, the electrode assembly is inserted in abattery case to prepare a battery. In the present invention, the lithiumsecondary battery may be a cylindrical shape using a can, a prismaticshape, a pouch shape or a coin shape, but is not particularly limited toits shape.

The second way is to insert the electrode assembly in a battery case andthen bring the battery case into thermocompression, to destroy thecapsules and impregnate the stored non-aqueous electrolyte solution intothe electrode assembly.

In this case, the thermocompression of the battery case may be carriedout under the conditions of 80° C. and 10 kgf/cm², but such conditionsmay be suitably modified depending on the glass transition temperatureand boiling point of the material to be used. This way also does notneed a separate process for introducing the non-aqueous electrolytesolution since the stored non-aqueous electrolyte solution is dischargedand impregnated into the separator and the electrodes, similar to thethermocompression of the electrode assembly.

Also, the third way is to carry out the thermocompression for destroyingthe capsules before and after the electrode assembly is inserted in abattery case. In this case, the electrode assembly is subject tothermocompression before being inserted in the battery case to destroy apart of the capsules provided in the electrode assembly, and after thethermocompressed electrode assembly is inserted in the battery case, thebattery case is further subject to thermocompression to destroy thecapsules which are left undestroyed, thereby impregnating thenon-aqueous electrolyte solution which have been stored in the capsulesinto the electrode assembly.

If the electrode assembly is directly subject to thermocompression, ahigh impregnation degree of the electrolyte into the electrode assemblymay be obtained in principle, but it is actually difficult to quantifythe solvent component of the electrolyte solution which evaporates afterthermocompression. Accordingly, in order to maintain the uniformity ofthe procedures, it is preferred to carry out the thermocompression aftersealing the battery case in which the electrode assembly is inserted.Also, in the case that several electrode assemblies are stacked, it ispreferred that the electrode assemblies are subject topre-thermocompression before stacking, and then the stacked electrodeassemblies are inserted in a battery case and are further subject tothermocompression.

Hereinafter, the present invention will be explained in more detail withreference to the following Examples. However, it should be understoodthat the Examples are provided for the purpose of illustrations only andto better explain to a person having ordinary skill in the art, and isnot intended to limit the scope of the present invention, so otherequivalents and modifications could be made thereto without departingfrom the spirit and scope of the present invention.

EXAMPLES Example 1

<Preparation of Capsules Having an Electrolyte Solution and a CoatingSolution Containing the Capsules>

A coating solution was prepared by using phase-separation and UV curingprocesses. First, ethylene carbonate and ethyl methyl carbonate weremixed in a volume ratio of 5:5, in which LiPF₆ was dissolved until 1mol/L of LiPF₆ solution was obtained, and polyethylene glycol diacrylate(PEGDA) 700 (Mn: 700) as a binder polymer was added to the solution sothat the weight ratio of PEGDA 700 and the solution was 1:1, to obtain afirst solution (Phase 1) as an electrolyte solution.

Then, in order to enhance the mechanical strength of the film ofcapsules for storing the electrolyte solution obtained above, Al₂O₃ asinorganic particles was added to the electrolyte solution of Phase 1 sothat the weight ratio of Al₂O₃ and the binder polymer is 1.2:1, followedby stirring and dispersing.

Meanwhile, 5.9 parts by weight of sorbitan monooleate (Span 80) as anonionic surfactant was added to 100 parts by weight of heptane, and 0.8parts by weight of MBF (benzene acetic acid) as a photoinitiator wasadded thereto, to obtain a second solution (Phase 2) in which heptanewas used as a main solvent. The first solution (Phase 1) and the secondsolution (Phase 2) were mixed in a ratio of 40:100, and the mixture wasagitated at a speed of 1,000 rpm or higher for 30 minutes or longer,followed by UV radiation with 365 nm UVA light. After UV radiation,heptane was evaporated to obtain capsules. The obtained capsules weresubject to SEM analysis, and the SEM image observed therefrom is shownin FIG. 2.

<Preparation of Battery>

In order to prepare an anode, 93 wt % of a carbon-based active material(MCMB 10-28, OSAKA GAS) and 7 wt % of polyvinyledene difluoride (PVDF)(Kynar 761, Elf Atochem) were dissolved in N-methyl-2-pyrrolidone (NMP)as a solvent and mixed in a mixer (Ika) for 2 hours, and the resultingsolution was coated on a copper foil as a current collector, followed bydrying at 130° C.

In order to prepare a cathode, 91 wt % of LiCoO₂, 3 wt % of PVDF (Kynar761) and 6 wt % of a conductive carbon (KS-6, Lonza) were dissolved inN-methyl-2-pyrrolidone (NMP) as a solvent and mixed in a mixer (Ika) for2 hours, and the resulting solution was coated on an aluminum foil as acurrent collector, followed by drying at 130° C. Meanwhile, as a barefilm for separator, a polypropylene-based separator (Celgard™ 2400) wasused.

The capsules obtained in the above, which contains an electrolytesolution, were coated in a certain amount on the surface of the anode byusing a coater (blade). After coating, a drying process was carried outunder vacuum at room temperature for 2 hours, thereby removing heptane.The dried anode, cathode and separator were inserted into an aluminumpouch to obtain a mono-cell. In a compressor whose temperature andpressure are controlled, a first pressing was carried out at 60° C. for10 minutes until the distance between the top plate and the bottom platebecomes 30 μm, so as to induce the destruction of the capsules at roomtemperature, thereby leading to the good wetting of the non-aqueouselectrolyte solution into the electrodes and the separator. Then, asecond pressing was carried out with raising the temperature to 80° C.for 20 minutes. At this time, the final temperature and pressure werecontrolled to 80° C. and 10 kgf/cm², respectively. The mono-cell thusfinally prepared was fully charged and evaluated for its performance.

Example 2

The procedure of Example 1 was repeated except that polyethylene glycolphosphate diacrylate obtained by mixing PEGDA 700 andbis[2-(acryloyloxy)ethyl]phosphate in a weight ratio of 1:1 was used asthe binder polymer before UV radiation, to prepare non-aqueouselectrolyte solution-containing capsules and a battery using thecapsules.

Comparative Example 1

The procedure of Example 1 was repeated except that the inorganicparticles of Al₂O₃ were not used, to prepare non-aqueous electrolytesolution-containing capsules and a battery using the capsules.

<Observation of Electrolyte in the Form of a Film>

In order to confirm that an electrolyte solution having a sphericalparticle structure is well changed in the form of a film throughthermo-compression between the cathode and the anode, the batteryprepared in Example 1 was observed by SEM image analysis. After thebattery in the form of a mono-cell was uniformly coated with anelectrolyte solution having a spherical particle structure by using acoater on the surface thereof, the surface of the anode was observedbefore and after the thermo-compression under the conditions of 80° C.and 10 kgf/cm², and the results thereof are shown in FIGS. 3 and 4. FIG.3 shows the SEM photograph of the anode surface beforethermocompression, and FIG. 4 shows the SEM photograph of the anodesurface after thermocompression. As can be confirmed from FIGS. 3 and 4,the electrolyte solution was formed in the form of a film on the anodesurface after thermocompression.

<Evaluation of Effect of Inorganic Particles>

After the batteries of Example 1 and Comparative Example 1 were fullycharged and stored at 85° C. for 4 hours, the thickness of the batterieswas measured and the results thereof are shown in FIG. 5. As can beconfirmed from FIG. 5, in Example where the capsules contain Al₂O₃ asinorganic particles, the electrolyte layer formed between the cathodeand the anode has increased mechanical strength to provide an effect ofpreventing the thickness expansion in the batteries when the batterieswere left at a high temperature.

Also, after the capsules having a non-aqueous electrolyte solution weredestroyed through thermocompression, their discharging voltage profilesat 1 C were observed and the results thereof are shown in FIG. 6, fromwhich batteries were confirmed to operate regardless of having or nothaving inorganic particles.

<Evaluation of Effect of Binder Polymer on Battery Performances>

In order to evaluate the effect of various binder polymers on batteryperformances, as in Example 2, polyethylene glycol phosphate diacrylate,a copolymer of polyethylene glycol diacrylate andbis[2-acryloyloxyethyl]phosphate having a phosphate (PO₄) functionalgroup capable of lowering the resistance on a surface of anode was usedas the binder polymer in the preparation of capsules, and the resultingcapsules were uniformly coated on the surface of the anode, followed bythermocompression, to prepare a battery having a layer where destroyedcapsules were dispersed.

The battery of Example 2 thus prepared and the battery of Example 1using polyethylene glycol diacrylate as a binder polymer were subject toa high rate discharge test, and the results thereof are shown in FIG. 7.As can be confirmed From FIG. 7, in the battery of Example 2 using abinder polymer where a phosphate (PO₄) functional group is present, ahigh rate discharging characteristic has improved as compared with thebattery of Example 1. Thus, in the preparation of capsules, a properfunctional group which is suitable to the anode (reduction) and thecathode (oxidation) can be introduced to the binder polymer, therebyenhancing the performances of the battery.

What is claimed is:
 1. A lithium secondary battery, comprising: anelectrode assembly having a cathode, an anode, and a separatorinterposed between the cathode and the anode; and a non-aqueouselectrolyte solution impregnated in the electrode assembly, wherein theseparator comprises a porous substrate and a layer having a plurality ofdestroyed capsules therein, the layer being bonded to at least onesurface of the porous substrate, and contacting the cathode or theanode, wherein each destroyed capsule has a film including a binderpolymer and inorganic particles, wherein binder polymer from films ofthe destroyed capsules bonds the layer to the porous substrate, andwherein the layer further comprises pores and micropores, wherein themicropores comprise interstitial volumes present between inorganicparticles, and wherein the pores comprise interstitial volumes presentbetween destroyed capsules.
 2. The lithium secondary battery accordingto claim 1, wherein the inorganic particles are selected from the groupconsisting of inorganic particles having a dielectric constant of 5 orhigher, inorganic particles having the ability to transport lithiumions, and a mixture thereof.
 3. The lithium secondary battery accordingto claim 2, wherein the inorganic particles having a dielectric constantof 5 or higher are selected from the group consisting of BaTiO₃,Pb(Zr,Ti)O₃ (PZT), Pb_(1-x)La_(x)Zr_(1-y)Ti_(y)O₃ (PLZT, 0<x<1, 0<y<1),Pb(Mg_(1/3)Nb_(2/3))O₃PbTiO₃ (PMN-PT), hafnia (HfO₂), SrTiO₃, SnO₂,CeO₂, MgO, NiO, CaO, ZnO, ZrO₂, SiO₂, Y₂O₃, Al₂O₃, SiC, TiO₂ inorganicparticles and a mixture thereof.
 4. The lithium secondary batteryaccording to claim 2, wherein the inorganic particles having the abilityto transport lithium ions are selected from the group consisting oflithium phosphate (Li₃PO₄), lithium titanium phosphate(Li_(x)Ti_(y)(PO₄)₃, 0<x<2, 0<y<3), lithium aluminum titanium phosphate(Li_(x)Al_(y)Ti_(z)(PO₄)₃, 0<x<2, 0<y<1, 0<z<3), (LiAlTiP)_(x)O_(y) typeglass (0<x<4, 0<y<13), lithium lanthanum titanate(Li_(x)La_(y)TiO₃,0<x<2, 0<y<3), lithium germanium thiophosphate(Li_(x)Ge_(y)P_(z)S_(w), 0<x<4, 0<y<1, 0<z<1, 0<w<5), lithium nitride(Li_(x)N_(y), 0<x<4, 0<y<2), SiS₂ type glass (Li_(x)Si_(y)S_(z), 0<x<3,0<y<2, 0<z<4), P2S5 type glass (Li_(x)P_(y)S_(z), 0<x<3, 0<y<3, 0<z<7)particles, and a mixture thereof.
 5. The lithium secondary batteryaccording to claim 1, wherein the binder polymer has a dielectricconstant of 1.0 to 100 (when measured at a frequency of 1 kHz).
 6. Thelithium secondary battery according to claim 1, wherein the binderpolymer is selected from the group consisting of polyethylene,polystyrene, polyvinylidene fluoride-co-hexafluoropropylene,polyvinylidene fluoride-co-trichloroethylene, polyethylene glycoldiacrylate, polyethylene glycol phosphate diacrylate, polyacrylate,polymethyl methacrylate, polyisobutylmethyl methacrylate, polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate,polyvinyl alcohol, polyethylene-co-vinyl acetate, polyethylene oxide,polyarylate, polyteterahydrofuran, polymethacrylic acid lithium,polyacrylic acid lithium, polymaleic acid lithium, polyvinyl sulfonicacid lithium, polyvinyl phosphonic acid lithium, cellulose acetate,cellulose acetate butyrate, cellulose acetate propionate,cyanoethylpullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose,cyanoethyl sucrose, pullulan, carboxymethyl cellulose, and a mixturethereof.
 7. The lithium secondary battery according to claim 1, whereinthe weight ratio of the inorganic particles to the binder polymer is inthe range of 1:1 to 10:1.
 8. The lithium secondary battery according toclaim 1, wherein the capsules store the non-aqueous electrolyte solutionbefore being destroyed.